The present invention relates to a primer composition, particularly one to be used for forming an abrasion-resistant organopolysiloxane coating layer excellent in adhesion, endurance, weathering resistance, and so forth on the surface of a molded article of a thermoplastic resin such as polycarbonate.
Although a thermoplastic resin, particularly, polycarbonate is excellent in transparency and impact resistance and is lightweight, so that it has been widely used as a structural material substituting for glass, it has disadvantages in that it is liable to be abraded and attacked by an organic solvent and that it undergoes changes with the lapse of time to cause discoloration and/or deterioration.
Therefore, it has been proposed to coat the surface of a molded article made of a thermoplastic resin with a thermosetting resin, and organopolysiloxane is believed to be particularly useful as such a thermosetting resin, because it can form a coating layer excellent in abrasion resistance and solvent resistance on the molded article. However, the organopolysiloxane coating layer is poor in the adhesion to the surface of a thermoplastic resin such as polycarbonate and in weathering resistance and endurance after being applied to such a surface, so that a process of applying a primer prior to the application of the organopolysiloxane coating has been proposed.
Known such primers include (1) thermoplastic acrylic primer (see Japanese Patent Laid-Open No. 138565/1977), (2) functional group (such as an amino group)-containing acrylic resin primer (see Japanese Patent Laid-Open No. 138476/1978), (3) silicone primer comprising a reaction product of an alkoxysilane having a functional group such as an amino or hydroxyl group with a cyclic acid anhydride (see Japanese Patent Laid-Open No. 81533/1978), (4) silicone primer comprising a mixture of a hydrolyzate of an epoxysilane with an aminosilane (see Japanese Patent Laid-Open No. 63176/1979), (5) acrylic primer comprising a functional group-containing thermoplastic acrylic resin and an ultraviolet absorber (see Japanese Patent Laid-Open No. 500809/1980), (6) acrylic primer comprising a thermosetting acrylic emulsion and an ultraviolet-screening compound (see Japanese Patent Laid-Open No. 160033/1980), (7) silicone primer comprising a reaction product among an aminosilane, an epoxysilane and an acid anhydride (see Japanese Patent Laid-Open No. 16573/1981), (8) thermoplastic acrylic primer comprising a reaction product among an acrylic monomer, an epoxy methacrylate and a hydroxybenzophenone ultraviolet absorber (see Japanese Patent Laid-Open No. 23661/1982), and (9) acrylic primer comprising a thermosetting acrylic resin containing a melamine crosslinking agent and an ultraviolet absorber (see Japanese Patent Laid-Open No. 53701/1985).
However, the application of these primers does not always gives a satisfactory effect. For example, the above primers (1) and (2) are each disadvantageous in that an active hydrogen group (such as an amino group) present therein absorbs moisture to result in lowered adhesion in the presence of moisture and that the weathering resistance can hardly be improved, though the adhesion to a substrate can considerably be improved. The primers (3) and (4) each have defects that the adhesion in the presence of moisture is insufficient owing to the presence of an active hydrogen group therein, that the primers tend to cause yellowing owing to the prsence of an amino group therein, thus rather lowering the weathering resistance in some case, that the hydroxyl and carboxyl groups contained therein easily react with the alkoxy group to cause the primer solution to be thickened with the lapse of time to make its application difficult, and that the degree of polymerization of the primer is increased to result in lowered adhesion though they considerably improve the adhesion to a substrate. The primer (5) is disadvantageous in that the long-term weathering resistance is yet insufficient though the weathering resistance is considerably improved, that when the primer is applied in an increased thickness for the purpose of further improving the weathering resistance, the polyorganopolysiloxane coating layer formed thereon is too soft to exhibit a sufficient abrasion resistance, and that when the amount of the ultraviolet absorber added is increased, the coating layer causes blushing or brings about poor adhesion. The primer (6) has a disadvantage due to its nature of an emulsion in that the usable ultraviolet absorber is limited from the standpoint of water solubility to fail in selectively using a highly effective ultraviolet absorber, and other disadvantages in that the weathering resistance cannot be improved to a satisfactory extent and that the adhesion tends to be rather poor, because the acrylic resin used is thermosetting. The primer (7) has defects similar to those of the primers (3) and (4). The primer (8) has defects that the preparation of an acrylic polymer containing a large amount of an ultraviolet absorber is difficult because of the radical stabilizing action of the ultraviolet absorber, so that it cannot sufficiently improve the weathering resistance and the adhesion. Further, the primer (9) has a defect that the adhesion cannot be sufficiently improved though the weathering resistance can be improved.
Accordingly, a principal object of the present invention is to provide a primer composition for forming an organopolysiloxane coating layer on a molded article made of a thermoplastic resin such as polycarbonate, which is free from the defects of the primers according to the prior art references, i.e., a primer composition which does not undergo any change with the lapse of time and permits the formation of an organopolysiloxane coating layer improved in adhesion and resistances to heat, hot water and weather.